1. Field of the Invention
The present invention relates to an operational amplifier circuit.
2. Description of the Related Art
FIG. 3 is a diagram illustrating a related-art operational amplifier circuit.
When a signal voltage is input to input terminals Vinn and Vinp, the signal voltage is modulated into a high frequency wave by a chopping circuit 81. The modulated signal voltage is input to an amplifier stage 82 and amplified. At this time, an input offset voltage of the amplifier stage 82 is also amplified simultaneously together with the modulated signal voltage. An output voltage of the amplifier stage 82 is input to a chopping circuit 83, and the signal voltage is demodulated and the input offset voltage of the amplifier stage 82 is modulated into a high frequency wave. The modulated offset voltage is chopping noise. A signal voltage output from the chopping circuit 83 is integrated by an integral circuit including an amplifier stage 84 and capacitors 85 and 86, to be a triangular wave. An output of the integral circuit is input to a switched capacitor notch filter 87. The switched capacitor notch filter 87 includes switches 93 to 100 and capacitors 101 to 103. A control clock of the switches 93 and 94 and the switches 99 and 100 and a control clock of the switches 95 to 98 are the same in frequency as a control clock of the chopping circuits 81 and 83, and have a relationship in which the waveforms are inverted.
Now, it is assumed that the signal voltage is a DC voltage and that the control clock of the chopping circuits 82 and 83 and a control clock of the switched capacitor notch filter 87 have a phase difference of 90°. In this case, the capacitor 101 and the capacitor 102 each hold the electric charges of the periodic signal voltage of the switched capacitor notch filter 87 at a constant point and transfer the electric charges to the capacitor 103. Thus, the electric charges stored in the capacitor 103 are always constant. Consequently, an input offset voltage component of the amplifier stage 82 is removed.
The signal voltage output from the switched capacitor notch filter 87 is amplified by an amplifier stage 88, and is added with a signal voltage amplified by an amplifier stage 80. In addition, the resultant signal voltage is amplified by an amplifier stage 89 to be an output voltage of the operational amplifier circuit. The input offset voltage of the amplifier stage 82 is removed, and hence, by using the operational amplifier circuit with feedback control, the input offset voltage of the operational amplifier can be regarded as being small. Further, in this case, the chopping noise caused by modulating the input offset voltage of the amplifier stage 82 is also removed by the switched capacitor notch filter 87.
The related-art operational amplifier circuit needs to generate a triangular wave by using the integral circuit including the amplifier stage 84 and the capacitors 85 and 86. However, this causes an increase in current consumption.